[meteorite-list] Rocks From Space Picture of the Day - November 5, 2007

Mon Nov 5 16:07:33 EST 2007

Dear Listees,

This is a very timely picture, thanks to most kindly to both Bernd Pauli and 
Michael Johnson's Rocks from Space Picture of the Day :-)

Just a click away and an interested person without the materials and 
microscope can vicariously participate with Bernd's clear and colorful image 
of his nice thin section of Lodranite/Acapulcoite material. Nice!

Bernd, could you kindly mention which of the grains were measured for those 
of us who want to confirm our little fun with a tape measure on our computer 
screens?

Another click away is David Weir's free Internet Meteorite Encyclopedia:
http://www.meteoritestudies.com/protected_LODRAN.HTM
http://www.meteoritestudies.com/protected_MONUMENT.HTM

there are insightful summaries on the current state of research into the 
evolution of these materials.  One could conclude from the information on 
David's site that there is a consensus among leading researchers that 
Acapulcoites and Lodranites were "identical" in their early history to 
believe them from a common precursor parent body, though the fragmentation 
history is believed to be varied.  Apparently, the grain size was originally 
thought to be indicative of the depth of the sample excavated in a simple 
model of the deeper it is the slower the solidification and interrupting of 
grain growth in size (thus, Acapulcoites propsed nearer to the surface, 
cooled quicker in space and had smaller grains.  This simplified concept has 
been put under the loupe most recently by Rubin, pointing to a likely more 
violent history of near anniliation impacts, re-assembling, and a final 
special delivery to earth, the last which liberated meteoroids from the 
mother lode less than 7 million years ago.  This is most exciting to me as 
4-7 million years is in the same blink of the Universe's eye we are living, 
and it gives hope that this can be sorted out and paired to something 
floating around space.

I hope this hasn't been already covered on the list (Martin,et. al., feel 
free to remind me), but would like to comment that if the current 
classification system is simply so lacking considering current knowledge, 
that a more modern approach like that of A. Rubin, T. McCoy, or C. Floss, 
authors Patzer _et. al._, that these grain sizes in the range that separates 
the classifications.  Perhaps, based on these comments a similar system to 
aqueous/thermal alteration H-L's nomenclature is motivated, only in this 
case instead of looking at aqueous alteration, it is primarily grain growth.

Rubin (Geochimica et Cosmochimica Acta, Volume 71, Issue 9, 1 May 2007, 
Pages 2383-2401) states:
"Acapulcoites and lodranites experienced moderate post-shock annealing, 
presumably resulting from burial beneath material of low thermal 
diffusivity. The annealing process repaired damaged olivine crystal 
lattices, lending acapulcoites and lodranites the appearance of unshocked 
(i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed 
during initial shock reverted to their low-pressure polymorphs during 
annealing. Some samples were subsequently shocked again; several 
acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the 
lodranite MAC 88177 reached S5."

To trace the evolution of thought on this issue, which touches on the 
formation and migration of the metal cores in asteroids, in the words of Dr. 
Christine Floss (2000) at Washington University:

"The simple bimodal classification of these meteorites based primarily on 
petrographic criteria, which has been used to date, appears to be inadequate 
to describe this diverse group of samples, as they represent a range of 
degrees of partial melting, both with and without accompanying melt 
migration. In some instances secondary processes on the parent body, such as 
cryptic metasomatism*, have further modified sample compositions."

quote abstracted from:
Floss C. (2000) Complexities on the acapulcoite-lodranite parent body: 
evidence from trace element distributions in silicate minerals. Meteorit. 
Planet. Sci. 35, 1073-1085.

*cryptic metasomatism: a fancy way to describe alteration (metasomatism) 
where the relative abundances of components of minerals changes but not into 
new minerals (cryptic).  Puzzling, for sure to work backwards through this.

Comments by Patzer, Hill and Boynton (2004) regarding Floss' statement:
"In view of the growing number of samples, however, this scheme has recently 
been suggested to prove too limited. We share this assessment as far as we 
can judge from our preliminary INAA results and we support an extended 
classification modus similar to that developed by [Floss].

Not to be forgotten was TimMcCoy's PhD thesis (1994) suggesting the Rosetta 
Stone Acapulcoite-Lodranite LEW 86220 and foreshadowing the entire subject:
"In one meteorite (LEW 86220), these basaltic, Fe, Ni, FeS-rich partial 
melts from a lodranite source region were injected into a cooler, 
acapulcoite region. The acapulcoite-lodranite parent body experienced a 
range of partial melting and melt migration. Cooling of this body may have 
been complex, with slow cooling at high temperatures, rapid cooling at 
intermediate temperatures, and slow cooling at low temperatures, indicating 
that the body may have broken up and gravitationally reassembled. One to 
three impact events liberated the acapulcoites and lodranites from their 
parent body. These meteorites represent our best opportunity to understand 
partial melting and incomplete differentiation of asteroids."

The other somewhat parallel example might be the H-E-D system, which also 
suffers by the one-dimensional classification that probably fails in some of 
the more interesting cases by using an arbitrary diogenitic content to 
separate polymict Eucrites from Howardites.  And not to forget the parallel 
situation with Winonaites, thankfully, being just one major class and sub 
classifications and a bunch of unofficial pairings.

Maybe the scientists who work out this classification system better are 
limited by access to material as 20 grams might not always be enough to go 
chondrule hunting in such complex cases.  After all, the 20 gram research 
retainer is arbitrary in itself is probably partly responsible for these 
classification issues in the first place, not to mention the practical 
problems relating to locality less pairings and the financial realities that 
affect our less than perfect world .  No solutions at this point, from me, 
except it would seem that scientists and dealers in these materials 
hopefully have a special friendship when classifying material that is 
destined to be cut up and distributed anyway, it seems that a few more thin 
sections from diverse portions of each locality, like Bernd's, could really 
be just what the doctor ordered ... and if all images were in a central 
library ...

Best health,
Doug

Just for fun, how would a classification system look for the Peruvian fall 
if we considered the sorting, mud and meteoritic powder content, 
atmospheric, aqueous and oxidative alteration of the yet to be announced 
main mass, powder, or fragment size and exterior shape, considering all of 
the terrestrial mixing, not to mention what happens to the pieces after they 
get into ten different Peruoid primary family members (primary dealers), and 
then a thousand different hands and mailboxes.  Not going to say more, but 
if I were meteorite collector from another planet, I think I know who's 
anomalous sample I'd want most.  All that from an impact hole...not to 
mention those containing llama hair spindles...

----- Original Message ----- 
From: <SPACEROCKSINC at aol.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Monday, November 05, 2007 6:15 AM
Subject: [meteorite-list] Rocks From Space Picture of the Day - November 
5,2007

> http://www.spacerocksinc.com/November_5_2007.html
>
>
>
>
> ************************************** See what's new at 
> http://www.aol.com
> ______________________________________________
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
> 